The present invention relates to prereforming and reforming of natural gas containing higher hydrocarbons along with methane. More specifically, the invention relates to a process that improves the overall efficiency of reforming plants with and without prereformers that have feed streams comprising steam, hydrogen, and natural gas containing higher hydrocarbons along with methane.
Steam Reforming:
The steam-methane reforming process is routinely used in the chemical processing industry to produce pure hydrogen or a synthesis gas comprising a mixture of hydrogen and carbon monoxide from natural gas. The reforming process is generally carried out at a high temperature and pressure to facilitate reaction between the steam and methane in the presence of a nickel catalyst supported on alumina or another suitable material.
Several improvements have been made in recent years to improve the overall process economics of the steam-methane reforming process. Specifically, research has focused on recovering waste heat from the process and developing coke resistant nickel-based catalysts. The most notable improvement in the reforming process has been the incorporation of a prereformer for converting (1) substantially all of higher hydrocarbons present in the natural gas to a mixture of methane, carbon oxides, and hydrogen and (2) a part of methane present in the natural gas to carbon oxides and hydrogen. A prereformer that is properly integrated into the reforming process can offer numerous benefits, including: (1) reducing the possibility of coke formation on the main reformer catalyst by converting most of the heavier hydrocarbons present in the feed stream; (2) reducing the load on the main reformer catalyst by converting a part of the methane present in the feed stream; (3) reducing the ratio of steam to natural gas required for the reforming reaction, (4) providing flexibility in processing natural gas feed from different sources; (5) providing a luxury of preheating the gaseous feed mixture to a higher temperature prior to introducing it into the main reformer; and (6) increasing the life of both the reforming catalyst and tubes. The numerous benefits of using a prereformer are widely known.
Prereforming:
Generally, the selection of prereformer operating conditions has been limited by the potential of carbon formation on the catalyst, which deteriorates both the catalyst particles and the catalytic activity, balanced with concerns for the efficiency of the prereformer itself. For a given natural gas feedstock containing higher hydrocarbons along with methane, the prereformer must be operated within a certain temperature window to avoid coke formation on the catalyst. It is well known that the catalytic activity will drop if the operating temperature is (1) above the upper temperature limit due to whisker-type carbon formation, and (2) below the lower temperature limit due to formation of gum-type carbon on the catalyst. Therefore, it is desirable to select an operating temperature that is neither too high nor too low to avoid deactivation of the prereforming catalyst by coke formation. It is also desirable from the thermal efficiency point of view to operate the prereformer at as high a temperature as possible without forming coke on the prereforming catalyst.
The development of a suitable catalyst has recently been a focus of prereformer technology. The nature of the catalyst used in the prereformer depends upon the composition of the gas feed mixture, which comprises a mixture of steam, hydrogen and natural gas containing higher hydrocarbons along with methane. More specifically, the commonly used nickel-based catalysts can only be used if there is some amount of hydrogen present in the feed gas mixture. It is well known that a nickel-based catalyst is inactive in an oxidized form for converting hydrocarbons including higher hydrocarbons and methane, and therefore must be reduced or activated with a reducing gas such as hydrogen gas to convert higher hydrocarbons and methane. On the other hand, a precious metal-based catalyst can be used to convert a majority of higher hydrocarbons present in the natural gas regardless of whether there is hydrogen present in the mixed gas feed mixture. This is because a precious metal-based catalyst is active even in the absence of a reducing gas such as hydrogen, and therefore does not require reduction or activation.
The operating condition and catalyst limitations discussed above, as well as efficiency concerns, have been the center of research regarding prereforming and reforming of natural gas. Various techniques to improve prereforming and reforming of natural gas have involved the use of different temperatures and pressures, different percentages of nickel and other materials in the catalyst, catalyst placement, varying steam to carbon ratios, and heat exchange with hot waste streams. See, e.g., U.S. Pat. Nos. 3,988,425; 4,104,201; 4,417,905; 4,631,182; 4,824,658; 4,919,844; 5,264,202; 5,773,589; and 5,932,141. However, despite these attempted improvements to the prereforming and reforming of natural gas, the reforming process still has problems with rapid deactivation of precious metal-based or nickel-based prereforming catalysts, probably due to coke formation, catalyst stability, or some other factors.
Partial Oxidation:
Partial oxidation is a well known method of producing a mixture of hydrogen and carbon monoxide. Although operating conditions, composition of feed gas mixture, and catalysts used in steam-methane reforming and partial oxidation processes are substantially different, a few common trends have emerged. Research has focused on the possibility of reducing high heat generation and coke formation in partial oxidation of natural gas processes by adding steam to the feed stream. Likewise, research has focused on the possibility of using oxygen to improve the overall performance of the steam-methane reforming process. See, e.g., EP 0936183 (adds 0.55 moles of oxygen per mole of hydrocarbon); EP 0982266 (steam added must be higher than the oxygen added to reaction mixture); and WO 99/48805 (adding oxygen containing gas, and optionally steam, in the amount of 0.55 to 0.90 moles of oxygen per mole of hydrocarbon).
Reforming of Methane with Carbon Dioxide:
Reforming of natural gas or methane with carbon dioxide can also produce synthesis gas, or a mixture of hydrogen and carbon monoxide. This process, however, is plagued by severe catalyst deactivation by coke formation. It is well known that this deactivation problem can be overcome by combining a partial oxidation reaction with the reforming reaction. Several papers have described the combination of the exothermic partial oxidation reaction with the reforming reaction to provide a thermally neutral process to produce synthesis gas. This process uses 0.25 to 0.6 moles of oxygen per mole of methane and a temperature ranging from 700 to 800° C.
Oxy-Steam Reforming of Natural Gas:
Synthesis gas can also be produced by oxy-steam reforming of natural gas. The amount of oxygen added in the oxy-steam reforming process varies from 0.10 to 0.50 moles of oxygen per mole of natural gas or methane at a temperature between 750 to 850° C. Although the impact of adding oxygen on overall conversion of methane and product selectivity in a steam-methane reforming process is widely known, the same is not true of information regarding selective conversion of heavier hydrocarbons with the addition of oxygen at prereforming temperatures that are considerably lower than 800° C. See, e.g., Hegarty et al., “Syngas Production from Natural Gas Using ZrO2-Supported Metals,” Catalysis Today, 42, 225-232 (1998) and Choudhary et al., “Simultaneous Steam and CO2 Reforming of Methane to Syngas Over NiO/MgO/SA-5205 in Presence and Absence of Oxygen,” Applied Catalysis, 168, 33-46 (1998).
U.S. Pat. No. 6,335,474 discloses a process for prereforming an oxygen-containing natural gas. According to this patent, a hydrocarbon feedstock with a content of higher hydrocarbons and oxygen is catalytically prereformed with a precious metal catalyst selected from Group VIII of the Periodic Table. The precious metal catalyst is claimed to be active in oxidation of hydrocarbons to carbon oxides and conversion of higher hydrocarbons to methane. While U.S. Pat. No. 6,335,474 teaches the use of a small amount of oxygen to activate the higher hydrocarbons without completely combusting them to carbon dioxide and water, or partially oxidizing them to carbon monoxide and hydrogen, there are several situations where the process would not be operable. For example, the patent is silent about using a non-noble metal catalyst. A non-noble metal catalyst, such as a nickel-based catalyst, would not work in the conditions described in the patent because of the absence of hydrogen. Furthermore, the method of this patent would not work if hydrogen were present because of preferential reaction of oxygen with hydrogen in the presence of a precious metal catalyst.
Accordingly, it is desired to provide a process that improves the overall efficiency of reforming plants with and without prereformers that have feed streams comprising hydrogen, steam, and natural gas containing higher hydrocarbons along with methane, wherein said process does not substantially suffer from the aforementioned deficiencies of other processes.
All references cited herein are incorporated herein by reference in their entireties.